Radon decay monitoring in air using characteristic emission of species in metal-assisted LIBS

Highlights

• The natural decay of radon is investigated by using the LIBS at characteristic lines of Cu(I).

• Time-resolving of the (Rn + air) and air plasma are carried out by using different time delays.

• The ionic peaks for N(II) and O(II) are studied in (Rn + air) plasma.

Abstract

Copper assisted optical emission spectroscopy is studied in the synthetic air and (Rn + air) environment. The copper plate acts as the metal target in the Q-SW Nd:YAG laser induced plasma. Several characteristic emissions are taken from oxygen, nitrogen and the copper species in (Rn + air) plasma within the control chamber. The emission lines due to the metal species and air components exhibit noticeably to be enhanced respect to those of the synthetic air. The corresponding amplitudes lucidly decrease during a ten day follow-up. The instantaneous reductions of the copper characteristic lines are in agreement with the radon decay. The plasma temperature is estimated based on the Boltzmann plot whereas the corresponding electron density is determined using the Stark broadening. It is shown that those plasma parameters gradually increase with time to reach the values of pure air after several radon half-lives.

Introduction

Radon is a colorless, odorless, radioactive noble gas which its most abundant isotope is ²²²Rn with half life of 3.8 days. The decay chain of radon is illustrated in Fig. 1. The radon detection attracts significant attention because it is a naturally radioactive gas from uranium or thorium decay chain (Nagda, 1994) used as an indicator in mine explorations and earthquake predictions. Moreover, the radon gas has been detected on the moon when a surge of alpha particles was identified during Apollo 15 mission mainly above Aristarchus plateau (Lawson et al., 2005). According to current explorations, there are attempts to investigate various methods for radon detection in Mars. For instance a preliminary map of atmospheric radon has been derived from analysis of ²¹⁴Bi and ²¹⁴Pb lines in the gamma ray spectra acquired by Mars Odyssey γ ray spectrometer (Meslin et al., 2015). Furthermore, the radiological hazard of radon is very important for human health elevating the risk of lung cancer (Frumkin and Samet, 2001).

There are four alpha sources i.e. ²²²Rn, ²¹⁸Po, ²¹⁴Po and ²¹⁰Po due to the decay of radon and its daughters within ²³⁸U chain reaction. Those radio-nuclides are simultaneously available during a single radon decay half life (Nagda, 1994).

In principle, the nuclear techniques for the collection and measurement of ²²²Rn in air can be subdivided into the active and passive methods. The former requires electric power and vacuum pump to collect the samples whereas the latter does not need any sophisticated equipments. An active method such as electronic-integration-device (EID) is usually used for the short-term measurements of radon. Conversely, the passive methods including solid state alpha-track-detector (ATD), polymeric-solid state nuclear track detectors (SSNTD), activated-charcoal-detector (ACD), and electret-ion-chamber (EIC) are employed to assess long term radon emission (NCRP, 1988, George, 1984, Zeeb and Shannoun, 2009).

On the other hand, the optical spectral emission (OES) resembles to be very useful technique analyzing various gaseous components in air. However, it occasionally encounters some difficulties mainly due to low resolution inspecting the multi-elemental spectra. Hence, the identification of emission lines ought to be carefully resolved (Kurt, 2007). The laser induced breakdown spectroscopy (LIBS) has found to be attractive for the elemental identification purpose. Unlike the other spectroscopic methods, it requires no preparation as to small amount of material is sufficient for sampling and analysis. This enables in situ or remote sensing via optical fiber allowing the measurement in hostile environments (Fichet et al., 2001, Parvin et al., 2013). Furthermore, the metal assisted technique accompanying simultaneous OES leading to the decomposition of gaseous components has been extensively investigated (Ghorbani et al., 2014, Maleki et al., 2015, Reyhani et al., 2012). Recently, the LIBS has been applied to determine the uranium concentration in the ore samples (Kim et al., 2012) and also specifying the uranium isotope ratio (Pietsch et al., 1998).

LIBS spectral enhancement due to the buffer gas addition in the control chamber has been recently reported (Abdulmadjid et al., 2006, Henry et al., 2007, Munadi et al., 2007, Pardede et al., 2005). Henry et al., investigated the role of helium addition on the analyte signal enhancement in laser-induced breakdown spectroscopy for the analysis of pure gaseous systems (Henry et al., 2007). Helium addition affects the plasma electron density via Penning ionization, as well as the initial plasma breakdown processes, the consequent plasma laser coupling and the energy absorption. Moreover, the contribution of helium for the enhancement of hydrogen signals in zircaloy alloys has been addressed (Abdulmadjid et al., 2006, Munadi et al., 2007, Pardede et al., 2005). The enhancement mainly arises from the metastable excited state of helium atom which is utilized to induce the delayed excitation of hydrogen atoms during ablation. On the other hand, the methods of preheating and reheating of plasma using dual lasers have been used to enhance the characteristic emissions at the expense of the background noise elevation (Babushok et al., 2006, Shoursheini et al., 2009, Shoursheini et al., 2010, Killinger et al., 2007, Antony et al., 2012).

Another method recently was employed to attest the spectral enhancement by means of excessive pre-ionizations due to the alpha particle emissions from the radioactive alpha emitter from gaseous radon radio-nuclide (Hashemi et al., 2014). The radioactive decay gives out the alpha particles at 5.48 MeV, which notably create excessive number of electron-ion pairs along its path in air inside the control chamber. It causes to increase the level of initial electrons with suitable electron energy distribution functions EEDF several thousand times higher than those available in the synthetic air that in turn facilitates the plasma ignition. In the laser induced plasma, the phenomenon contributes to populate further atomic transitions. As a consequence, it gives rise to somewhat more intense characteristic emission profile (Hashemi et al., 2014).

Here, the optical characteristic emissions of species are obtained during laser induced plasma to investigate the significant differences of the plasma properties in (Rn + air) and the atmospheric air environments. The follow-up of metal characteristic emission emphasizes the fact is strongly dependent on the Rn decay. We have previously shown that the corresponding emission intensities due to the metal species i.e. Pb, Zn, Cu, and those of air components (nitrogen and oxygen) in the presence of radon trace are sensibly enhanced compared to those in pure air (typically, Pb(I): 143%, Zn(I): 93%, Cu(I): 55%, N(I) and O(I): 180%). (Hashemi et al., 2014). Furthermore, the ionic species are identified to be an indicative for (Rn + air) whereas such species are not found by inspecting spectra in the pure air. The instantaneous decay of the radon are monitored during a 10 day follow up of the metal characteristic emissions via daily measurements of the plasma emissions using the plasma ignitions by means of the successive laser shots.